In a typical nuclear reactor power plant, an example of one design of such being shown in FIG. 1, the reactor pressure vessel utilized to generate heat, as well as other components such as steam generators, pumps, pressurizer, and associated piping, are housed in a containment building. Typically, a containment building may be made of concrete, stainless steel, or other appropriate material. The containment building defines a refueling cavity and completely encloses the entire reactor and reactor cooling system (RCS) and insures that an acceptable limit for release of radioactive materials to the surrounding/local environment would not be exceeded, even in the unlikely occurrence of a gross failure of the RCS. All operations and procedures associated with the functioning of the reactor vessel and RCS are performed within the containment building.
Typically, the refueling cavity is a split-level area, wherein the upper level contains a reactor cavity and the lower level consists of a fuel transfer canal. The reactor vessel is housed within the reactor cavity which is also a reinforced concrete structure. When filled with water for refueling, it forms a pool above the reactor within the refueling cavity. The refueling cavity is filled to a depth that limits radiation at the surface of the water, usually up to an operating deck from which maintenance procedures are conducted, to acceptable levels. Typically, the water is in the form of borated water, which helps to minimize exposure levels. The water provides an effective and transparent radiation shield for personnel on the operating deck, as well as a reliable medium for removal of decay heat from the reactor vessel. During refueling, spent or depleted fuel is removed from the reactor core, transferred under water, and placed in the fuel transfer system by the plant's refueling machine; and new or fresh fuel is similarly transferred from a fuel transfer building and loaded therein. During this down time, other maintenance or inspection procedures can also be performed. In order to perform these procedures, certian components within the reactor vessel need to be removed from its interior.
The main components within the reactor vessel apart from individual fuel assemblies, are the reactor upper and lower internals structures. The internals structures are designed such that they can be completely removed from the vessel. This facilitates the in-service inspection of the internals and provides access for inspection of the entire inner surface of the vessel. These structures are stored within the refueling cavity during these maintenance procedures.
The reactor internals structures are designed to support and position the reactor core fuel assemblies and control rod assemblies, and to provide a passageway for the reactor coolant and support in-core instrumentation. An exemplary reactor vessel lower internals assembly may consist of a core barrel, core baffle, lower core support plate and support columns, thermal shield, and the intermediate diffuser plate and bottom support casting. Within the core barrel are the axial baffle and former plates which are attached to the core barrel wall, and form the enclosure periphery of the assembled core. The lower core support plate is positioned at the bottom level of the core below the baffle plates and provide support and orientation for the fuel assemblies. The lower core support plate is perforated and contains fuel assembly alignment pins on its upper surface for the proper orientation of the fuel assemblies. The lower core support structure also serves to provide passageways and control for the coolant flow.
An exemplary reactor vessel upper core support structure or internals assembly includes the top support plate, the upper core plate, support columns, and control rod guide tubes. The main functions of the reactor vessel upper internals are to align and locate the upper ends of the fuel assemblies also with fuel assembly alignment pins which project downward from the bottom of the upper core plate, and to guide the control rod cluster and associated drive shafts. The guide tube assemblies sheath and guide the control rod drive shafts in control rods. The control rod drive shafts remain with the upper internals as they are removed from the reactor core.
During a typical maintenance procedure, such as refueling or inspection of the interior of the reactor vessel, the reactor is first shut down and cooled to ambient conditions. Then the reactor vessel closure head studs and nuts are removed from the reactor vessel and guide studs are installed in, typically, three selected stud holes. The remainder of the stud holes are plugged. The guide studs serve as a means for proper orientation of fuel assemblies and the reactor internals, as well as reinstallation of the reactor vessel head assembly. As the reactor vessel head is raised by the plant's overhead polar crane, the reactor cavity is filled with borated water to the level of the vessel flange. The head is slowly lifted while water is continually pumped into the cavity; the water level and vessel head being raised simultaneously. The reactor vessel head is finally removed to a dry storage area within the containment building. The reactor vessel upper internals, and associated control rod drive shafts, are then removed. The reactor internals are removed by a reactor internals lifting device, which consist of a structural frame suspended from the overhead crane. The frame is lowered onto the guide tube support plate of the internals and manually bolted to the support plate. Bushings on the frame engage the guide studs to provide the precise guidance during removal and replacement of the internals package. The upper internals and associated control rod drive shafts are lifted out of the vessel and stored in the underwater storage stand in the refueling cavity. The fuel assemblies are now free from obstructions and are ready to be moved from the reactor core. When necessary, the lower internals structures are also removed in the same manner and placed upon their respective storage stand.
The refueling cavity is large enough to provide storage space for the reactor upper and lower internals (shown in phantom in FIG. 1), and for the miscellaneous tools used for the refueling and maintenance. The internals structures are stored on typically stainless steel structural fixtures installed in the refueling cavity. The upper internals storage stand is used to support the upper internals structure from its top flange when removed from the reactor vessel. The construction of the internals does not readily permit them to be totally supported from the bottom in a conventional manner and are therefore additionally supported from above. The stand, as well as the upper internals, remain completely underwater during the maintenance procedures. The reactor vessel lower internals are likewise stored upon a storage stand constructed within the refueling cavity. The lower internals however can be stored and supported by completely resting on the lower core support plate. These storage stands are typically designed in the form of a welded and bolted box section. In order to construct these support structures, they must be put in place after the containment building has been constructed. Due to the design of the storage stands, their construction is very expensive and time consuming. Such a large stainless steel structure can be very hard to fabricate, and therefore requires specialized tooling and training. Extensive drilling is typically required to assemble the stands inside the containment building. Also these structures must be heavily supported laterally so as to prevent any damage to the internals structures while stored thereon. They must also be positioned within the refueling cavity in such a manner so as to not interfere with refueling, or other maintenance operations. Great care must be taken not to damage the upper and lower internals structures while stored within the refueling cavity during the transfer of fuel assemblies out of and into the reactor vessel core.